The above-mentioned application Ser. No. 08/709,062, filed on Sep. 6, 1996 by the present inventor, discloses a ballast circuit using regenerative gate drive circuitry to control a pair of serially connected, complementary conduction type switches of an d.c.-to-a.c. converter. Such switches may comprise an n-channel enhancement mode MOSFET and a p-channel enhancement mode MOSFET, for example. In the disclosed ballast, the phase angle between a resonant load current and a control voltage for the switches moves towards 0.degree. during lamp ignition, providing reliable lamp ignitiion.
It would be desirable to adapt the foregoing ballast to allow a user to adjust the intensity of lamp output while the lamp is operating. For lamps having resistively heated cathodes, it would also be desirable to provide, upon initial power delivery to the ballast, a cathode preheat period during which the cathodes are heated to a desired temperature before igniting the lamp.